1. Field of the Invention
The present invention relates to the field of fluid separation. More specifically, the present invention relates to the separation of oil and water in connection with hydrocarbon production activities.
2. Background of the Invention
Effective separation of water from produced crude oil is a continuing need for the oil industry. Effective separation is particularly advantageous during the early stages of production of a well when there may be high water content. Even in wells that do not have significant initial water production, water cuts can increase over the life of a well to the point where the production fluids have to be treated to remove water.
When water is produced with oil it is frequently in the form of an emulsion. An emulsion is a heterogeneous liquid system consisting of two immiscible liquids, with one of the liquids being intimately dispersed in the form of droplets in the second liquid. The matrix of an emulsion is called the external or continuous phase, while the portion of the emulsion that is in the form of small droplets is called the internal, dispersed, or discontinuous phase.
In most emulsions of crude oil and water, the water is finely and spherically dispersed in the oil. This is referred to as a water-in-oil emulsion. The spherical form of the water droplets is a result of interfacial tension (IFT), which forces the water to present a minimum surface area to the oil.
The stability of an emulsion is controlled by the type and amount of surface-active agents present. In some instances, particularly with heavy oils, finely divided mineral solids existing within the production stream can act as emulsifying agents. The emulsifying agents form interfacial films around the droplets of the dispersed phase and create a barrier that slows down or inhibits coalescence of the water droplets.
The tendency of heavy oils to contain water-in-oil emulsions is attributable to the presence of certain hydrocarbon molecules sometimes found in heavy crudes. Particularly, asphaltenes and high naphthenic acids in heavy crudes tend to form stable, water-in-crude oil emulsions. The polar naphthenic acids and asphaltenes in the crude oil along with sub-micron size solids, such as silica, clay, and other minerals, undesirably stabilize heavy crude petroleum emulsions.
Crude oil dehydration treating systems are typically used to reduce the basic sediment and water (or “BS&W”) of crude oil to a certain acceptable level specified by a crude oil purchaser such as a pipeline company. The level of sediment and water typically specified by purchasers is less than 1% by volume. In particular, with bitumen produced from oil sands, both water and solids result from the oil sands extraction process. This means that solids have to be separated from the crude oil.
It has been known to separate water from crude oil in storage tanks using mechanical separators and gravitation. However, when water forms a stable emulsion with heavy crude oil, the use of storage tanks and mechanical separators may be difficult. This is particularly true with emulsions of heavy oil and water produced from a reservoir formation. Such crude oil fluids can contain from about 1% to about 60% water by volume. A common range of emulsified water in crude oil heavier than 20° American Petroleum Institute (API) is from 10% to 35%.
In an effort to further separate produced water from crude oil, it is also known to treat the well stream with chemicals. These chemicals are referred to as dehydration chemicals or demulsifiers. Various chemical additives have been used with some effect in treating water-in-oil emulsions. Commercially available chemical demulsifiers such as ethoxylated-propoxylated phenolformaldehyde resins and ethoxylated-propoxylated alcohols are used for demulsification of crude oils. Demuslifiers counteract the emulsifying agent, allowing the dispersed droplets of the emulsion to coalesce into larger droplets and settle out of the matrix. However, the effectiveness of these demulsifiers on heavy crude oils, particularly those containing asphaltenes, naphthenic acids and inorganic solids, may be limited.
U.S. Pat. No. 6,491,824 discloses the treatment of sludge emulsions. Various “emulsion breakers” are listed, including dodecylbenzylsulfonic acid (DDBSA), the sodium salt of xylenesulfonic acid (NAXSA), epoxylated and propoxylated compounds, anionic cationic and nonionic surfactants, and resins such as phenolic and epoxide resins. Additional examples of demulsifiers are disclosed in U.S. Pat. No. 1,500,202; U.S. Pat. No. 2,290,411; U.S. Pat. No. 2,568,741; U.S. Pat. No. 2,324,492; U.S. Pat. No. 3,553,149; U.S. Pat. No. 4,160,742; U.S. Pat. No. 4,686,066; and U.S. Pat. No. 4,738,795.
Where the crude oil is heavy oil, it is common to also employ gravity and electrostatic separators. Gravity settling and centrifugation in conjunction with chemical demulsifiers have also been employed. It is also known to treat the heavy oil with light oil or distillate along with the demulsifier. In some instances, demulsifiers are formulations containing about 50% weight (wt.) of a carrier solvent and 50% wt. of active demulsifying ingredients. The ingredients are commonly demulsifier molecules that are linear or branched alkyl chain ethoxylated alcohols.
In some cases, known technologies for the separation of water from heavy oil result in an intermediate emulsion rag layer. Further processing of the rag layer can be useful to recover the crude oil and discharge the water. The problem is faced both at production facility separators and in refinery oil/water separators. Recently, a microwave technology has been disclosed in U.S. Pat. Nos. 6,086,830 and 6,077,400 which discuss the use of microwaves for treatment of hard-to-treat emulsions.
Regardless, improved demulsifiers for heavy crude oil emulsions and for bitumen emulsions are needed. Also, a need exists for a new additive that reduces the rag layer. Further, a need exists for a method of demulsifying a water-in-oil emulsion using a salt of a polynuclear aromatic sulfonic acid.